1. Field of the Invention
The present invention relates to a process of the formation of heteroepitaxy including a process of preprocessing of a surface, and more particularly to a process of the formation of heteroepitaxy for forming a GaAs-on-Si substrate by growing a GaAs film on a silicon substrate after preprocessing a surface of a silicon substrate.
2. Description of the Prior Art
GaAs is used as a compound semiconductor substrate for forming a high speed transistor such as a HEMT.
GaAs-on-Si in which a GaAs film is formed on a silicon substrate has attracted attention in recent years as such a substrate. Since GaAs-on-Si has high mechanical strength as compared with a GaAs bulk substrate and is easy to obtain a large diameter, it is easy to handle and suitable for mass production. Therefore, there is a big demand for practical use for the process of the formation of GaAs-on-Si. However, a GaAs grown film formed by an MOCVD method or the like has defects in many cases since crystal lattice constants of the Si substrate and the GaAs grown film are different from each other (Si: 5.4307 .ANG., GaAs: 5.6537 .ANG.). Accordingly, a process of the formation of heteroepitaxy which solves such a problem and is capable of aiming at improvement of crystallinity of a GaAs grown film is being demanded.
FIG. 1 is a flow chart for explaining a process of the formation of heteroepitaxy according to a prior art, which is a process of forming a GaAs film on a Si substrate by an MOCVD method.
First, the pressure is decreased after placing a Si substrate on a placing table in a chamber of an MOCVD apparatus. The Si substrate has a (100) plane being 2.degree. off toward [011].
When a predetermined pressure is reached, hydrogen (H.sub.2) gas having a flow rate of 12 SLM and arsine (AsH.sub.3) gas having a flow rate of 0.05 SLM are introduced into a chamber, thus maintaining the pressure at 76 Torr. Furthermore, in order to remove a natural oxide film on the surface of the Si substrate, heat processing is applied for 10 minutes under the condition of the substrate temperature at 1,000.degree. C. At this time, since the circumference of the silicon substrate is covered by AsH.sub.3 gas, particles remaining on an inner wall of the MOCVD apparatus is prevented from reaching to the Si substrate.
Next, in order to form a GaAs initial film which becomes a growth nucleus on the grown plane of the Si substrate, hydrogen (H.sub.2) gas, arsine (AsH.sub.3) gas and hydrogen gas containing trimethylgallium (TMG) are introduced into the chamber after the substrate temperature is lowered to 400.degree. C. When this state is maintained for a predetermined period of time, the GaAs initial film having a predetermined film thickness is grown on the Si substrate.
Then, the temperature of the Si substrate is raised and maintained at 600.degree. C. In succession, H.sub.2 gas, AsH.sub.3 gas and H.sub.2 gas containing trimethylgallium (TMG) are introduced into the chamber. When this state is maintained for a predetermined period of time, a GaAs single crystal layer having a predetermined film thickness is grown. With this, a heteroepitaxy having a GaAs-on-Si structure is fabricated.
However, it has become clear that unevenness on the surface of the GaAs single crystal layer is large when the surface of the GaAs single crystal layer grown by the above-mentioned process is observed by atomic force microscopy (AFM). Further, it has also become clear that the surface of the GaAs single crystal layer is etched and there are many lamination defects on the surface of the GaAs single crystal layer when etch pits are observed.
When densification and micro-patterning of a semiconductor device are demanded, the existence of unevenness and crystal defects on the surface of the GaAs single crystal layer becomes a serious obstacle in element fabrication. For example, unevenness of the surface becomes an obstacle in patterning, and crystal defects cause deterioration of characteristics such as increase of a leak current of a transistor and decrease of life time.
Besides, a preprocessing process of heat treatment of a surface of a Si substrate in the AsH.sub.3 /H.sub.2 gas mixture has been known, but such a problem that a Si substrate surface becomes rough when heat treatment is performed at high temperature of 1,000.degree. C. is described in Applied Physics Letter 59(26), Dec. 23, 1991, pp. 3458-3460.
Further, there is such a preprocessing process of heat treatment at a low temperature in gas composed of a chloride of a IIIB group element. This is a process of removing a natural oxide film existing on the surface of a silicon substrate before the growth of a GaAs film through a chemical reaction with gas composed of chloride of a IIIB group element, in which the silicon substrate is exposed to the gas so as to have chlorine and silicon react to each other and gallium and oxygen react to each other so as to evaporate the natural oxide film.